how fast can you finish a game? what makes it take longer?

If you're asking "How long will the first game take?" I'd recommend using the "half an hour per player, plus half an hour" rule.  It's true that Dust isn't terribly complicated, but the card special abilities are not really obvious from the images alone, so there's a learning curve there, and depending on who you play with, people can get really anxious about where they should deploy, move, etc.  With some familiarity with the rules, I think you'll find the game paces itself quite nicely, since you're not fighting over victory points per se, but over the rate at which you gain victory points, so there's an inexorable pull toward conclusion that gives a nice feeling of tension throughout.

Hawknight said:

Well if you ask hiow long does a usual Dust game last then it's something like 2-3h (all depends on the number of player and their expirience). The most time consuming part is planning. First thing and the most important each turn is choosing your card. It will allow you to do few things but with much limitation. You can never plan a massive advance cause you will get a card with much attack and just a little of movement / production. Sometime it takes like 5-10min per player to plan everything and then it's 2-3min to execute it all including their fights!

You can make some kind of "time restrictions" - 5min for each player to do their entire turn (excluding/including the fights) but that would hurt your game.

00zaphod I recommend that you make/find/copy a card clarification sheet for each player (made one myself an that helps ppl alot).